This invention relates to irradiance of a workpiece, and more particularly to producing a desired spatial temperature distribution across a workpiece.
The manufacture of a semiconductor involves a number of thermal cycles, in which a wafer, typically silicon, is heated from room temperature to a high temperature such as 900xc2x0 C., for example. Significantly higher or lower temperatures may be required depending upon the particular application. The wafer is heated relatively quickly, with a typical ramp rate of at least 100xc2x0 C. per second.
During such heating cycles, it is critically important that all points on the wafer remain at a uniform temperature relative to one another. If the temperature distribution across the wafer is non-uniform, thermal gradients will cause the crystal planes within the wafer to slip, thereby breaking the crystal lattice. A very small spatial movement, on the order of 0.2 xcexcm, may completely destroy the crystal lattice. Thermal gradients may also cause other damage, such as warpage or defect generation. Even in the absence of slippage, a non-uniform temperature distribution across the wafer may cause non-uniform performance-related characteristics, resulting in either inadequate performance of the particular wafer, or undesirable performance differences from wafer to wafer.
Thus, the industry defines a xe2x80x9cprocess windowxe2x80x9d, which is an acceptable temperature range in which the temperature of each portion of the wafer must be kept in order to maintain performance goals. In the past, a non-uniformity of no more than xc2x110xc2x0 C. across the wafer at all times during the thermal cycle was acceptable.
However, with the manufacture of increasingly high performance semiconductor computer chips, and as larger numbers of device features are required on increasingly compact chips, an increasingly uniform temperature distribution across the wafer is required at all times throughout the thermal cycles, i.e. both during ramp and at a process temperature, which is usually a constant temperature. Industry roadmaps indicate that for devices with 0.25 xcexcm spacing, at a process temperature of 1100xc2x0 C. a temperature uniformity of xc2x13xc2x0 C. (i.e. 3xc2x0 C.=3xcex8 where xcex8 is the standard deviation of the temperature distribution across the wafer) will be required, and temperature uniformity of xc2x11xc2x0 C. (3xcex8) will be required for devices with 0.18 xcexcm spacing.
In addition, faster ramp rates, on the order of 400xc2x0 C. per second or higher, will be desired in the near future.
Conventional rapid thermal processing (RTP) techniques do not appear to be capable of achieving either the required degree of uniformity or the desired ramp rate.
One example of a conventional RTP technique includes rotating a wafer, and heating the wafer with a large number of tungsten-halogen lamps, each of which channels radiation toward the wafer surface through one of a large number of light pipes. Wafer temperature is measured with a comparatively small number of stationary pyrometers, each of which measures radiation thermally emitted by the wafer. Each measuring pyrometer is located at a different radial distance from the centre of the rotating wafer, so that the resulting temperature profile describes the average temperatures around a number of annular rings of the wafer, each annular ring corresponding to the radial distance of a particular measuring pyrometer. The resulting temperature versus time profile is then entered into a control computer, which employs a number of feedback control loops to control the power to the individual lamps or group of lamps associated with each pyrometer or sensor.
This technique has a disadvantage, in that it lacks the ability to detect or correct for temperature differences between any two points lying in the same annular ring, due to the constant rotation of the wafer relative to the pyrometers. Thus, while this technique is able to maintain a number of annular rings at relatively uniform average temperatures, it is not capable of either detecting or correcting for circumferential temperature differences. A mere 1% variation of absorption from one side of the wafer to the other may cause more than a 30xc2x0 C. temperature variation at 1050xc2x0 C. Thus, this technique is not suitable for the current industry requirements.
Also, to ensure accurate measurements, the plurality of pyrometers must be carefully calibrated, resulting in additional time and effort.
Modifying this technique for a non-rotating wafer would require a large increase in the number of pyrometers, which would lead to serious calibration difficulties, in addition to the added expense and difficulty of designing the hardware and software required to accommodate a large plurality of pyrometers and related control loops.
A further difficulty arises from reflection, by the walls of the process chamber, of radiation reflected or thermally emitted by the wafer. Such reflections may heat the wafer in a non-uniform manner, and may also produce measurement errors.
The substitution of a camera or CCD in this technique would not be practical, partly because the process hardware tends to obscure the view of the wafer, and partly because a camera or CCD would be particularly susceptible to errors induced by internally reflected radiation.
Furthermore, the use of a plurality of heat sources requires manual calibration of each such heat source, with the result that simple replacement of a burnt-out bulb may become a tedious and time-consuming process.
Moreover, the spectral distribution of tungsten-halogen heat sources may pose additional undesirable effects. Tungsten irradiance sources typically produce only 40% of their spectral energy below the 1.2 xcexcm band gap absorption of room-temperature silicon, resulting in an inefficient thermal cycle. Also, the wavelengths generated by tungsten sources may be sufficiently long to penetrate through a substrate side of the wafer and be non-uniformly absorbed by highly-doped features on a device side of the wafer, resulting in an increasingly non-uniform temperature distribution. Such an effect may be aggravated in devices involving insulating layers such as silicon on oxide (SOI). Irradiance fields produced by tungsten sources may be red-shifted as the power supplied to the source is decreased, resulting in even greater inefficiency and greater penetration of radiation into the device side.
In addition, as the temperature of silicon increases, it is able to absorb increasingly longer wavelengths of radiation. Thus, hotter areas of the wafer may absorb greater amounts of energy at the longer wavelengths produced by tungsten-halogen sources than cooler areas of the wafer, resulting in faster heating of the hotter areas and thermal runaway.
An additional problem arises from the slow thermal time constants of tungsten lamps. Fast ramp rates to desired process temperatures require fast feedback controls. For example, heating at 500xc2x0 C./sec to a process temperature with a uniformity of xc2x11xc2x0 C. ideally requires a response time of xc2x12 ms (xc2x11xc2x0 C./500xc2x0 C./sec), whereas tungsten lamps typically have much longer response times of fractions of a second.
Finally, this technique does not appear to be capable of achieving a ramp rate of 400xc2x0 C. per second which will soon be desired.
Thus, there is a need for a better heating device for semiconductor processing.
Specific embodiments of the current invention address the above need by dynamically producing a high-resolution spatially resolved temperature profile of the temperature distribution across an entire surface of a workpiece throughout a thermal cycle, and using this spatially resolved temperature profile to produce and maintain a desired temperature distribution at all points across the surface, at all times during the thermal cycle.
In accordance with one aspect of the invention, there is provided a method and an apparatus for producing a desired spatial temperature distribution across a workpiece. The method includes irradiating a plurality of areas on a surface of the workpiece to create localized heating of the workpiece in the areas, to produce the desired spatial temperature distribution in the workpiece. Preferably, irradiating includes exposing each one of the plurality of areas to radiation to produce the localized heating. The method may further include producing a representation of an instantaneous spatial temperature distribution in the workpiece, and producing an instantaneous spatial temperature error distribution as a function of the desired spatial temperature distribution and the instantaneous spatial temperature distribution.
Preferably, the method includes absorbing radiation exitant from the surface. Producing the representation may include producing at least one signal representative of radiation intensity from the surface. The method may further include controlling the amount of the localized heating by irradiating in response to the instantaneous spatial temperature error distribution. Optionally, exposing includes directing radiation from at least one energy source to the surface, and selectively varying, as a function of the representation, a variable opacity of each of a plurality of filter portions of a filtering member interposed between the at least one energy source and the surface. The apparatus includes means for carrying out the method.
In accordance with another aspect of the invention, there is provided a system for producing a desired spatial temperature distribution across a workpiece. The system includes a locator for locating the workpiece in a desired position relative to an energy source, and an irradiance system for irradiating a plurality of areas on a surface of the workpiece to create localized heating of the workpiece in the areas, to produce the desired spatial temperature distribution in the workpiece. Preferably, the system includes a processor circuit in communication with the irradiance system, and the processor circuit is programmed to control the irradiance system to expose each one of the plurality of areas to radiation to produce the localized heating. The irradiance system may include a measuring system for producing a representation of an instantaneous spatial temperature distribution in the workpiece. Optionally, the processor circuit is programmed to control the measuring system to produce an instantaneous spatial temperature error distribution as a function of the desired spatial temperature distribution and the instantaneous spatial temperature distribution. Preferably, the system further includes a radiation absorbing environment for absorbing radiation exitant from the surface.
The measuring system preferably includes an imaging system. The imaging system may include a charge-coupled device, and the processor circuit may be programmed to control the charge-coupled device to produce at least one signal representative of the surface.
Preferably, the irradiance system includes at least one energy source, which may be an arc lamp, for directing radiation to the surface. The irradiance system may further include a filtering member interposed between the at least one energy source and the surface, the filtering member having a plurality of filter portions, each of the plurality of filter portions having a variable opacity, and the processor circuit is programmed to selectively vary, as a function of the representation, the variable opacity of each of the plurality of filter portions, thereby producing the desired spatial temperature distribution in the workpiece.
When applied to a semiconductor wafer as a workpiece, the preferred measurement system has a spatial resolution finer than the smallest thermal scale length in the system and a time response faster than the shortest system time constant. Preferred embodiments of the invention employ a minimal number of measurement devices and a minimal number of heat sources, thus avoiding calibration difficulties and added expenses. Such embodiments also minimize the effects of reflection by the chamber walls of radiation emitted or reflected by the wafer, thereby minimizing an additional source of non-uniform heating of the wafer and also minimizing a source of measurement error which would otherwise interfere with the ability to produce the desired temperature distribution. A short-wavelength arc lamp may be employed as a primary irradiance source, resulting in highly efficient absorption in a thin surface of the substrate side of the wafer, with virtually no penetration of the radiation into the device side. Finally, embodiments of the current invention capable of producing ramp rates on the order of 400xc2x0 C. per second or even higher may be constructed. Thermal time constants of less than 1 ms for arc lamps make control of these fast ramps possible.
In addition to producing a uniform temperature distribution throughout a thermal cycle, embodiments of the invention may just as easily be used to produce any particular desired non-uniform temperature distribution, or to produce a dynamically changing series of desired temperature distributions.
Further aspects of the present invention will be apparent to one of ordinary skill in the art upon reviewing the specific embodiments described in the following detailed description and accompanying drawings.